1. Field of the Invention
The invention relates to X-ray tubes with rotating anodes wherein the suspension system of the anode has only passive magnetic bearings. More particularly, in such tubes, it relates to a device for compensating the weight of the assembly formed by the anode and the rotor.
2. Description of the Prior Art
X-ray tubes, for example those used for medical diagnosis, are generally constituted (FIG. 2) like a diode, i.e. with a cathode 21 and an anode 22 or anti-cathode. These two electrodes are enclosed in a vacuum-tight casing 8 that enables electrical insulation to be set up between them. The cathode 21 produces a beam of electrons 23 and the anode 22 receives these electrons on a small surface area forming a focal spot from which the X-rays are emitted.
When a high supply voltage is applied by a generator 24 to the terminals of the cathode 21 and of the anode 22 in such a way that the cathode is at the negative potential -HV, a current called anode current is set up in the circuit through the generator 24 providing the high supply voltage. The anode current goes through the space between the cathode and the anode in the form of the beam of electrons 23 which impinge on the focal spot.
A small proportion of the energy dissipated to produce the beam of electrons 23 is converted into X-rays. The rest of this energy is converted into heat. Because of this fact, and also because of the substantial instantaneous power values (in the range of 100 KW) brought into play, and the small dimensions (about one millimeter) of the focal spot, it has long been the practice for manufacturers to make X-ray tubes with rotating anodes where the anode is made to rotate in order to distribute the heat flux over a crown or ring called a focal ring, with an area that is far greater than that of the focal spot, the usefulness of this structure being all the greater as the rotation speed is high (generally between 3,000 and 12,000 rpm).
A standard type of rotating anode has the general shape of a disk with an axis of symmetry 4 about which it is made to rotate by means of an electrical motor 1. The electrical motor has a stator 2 located outside the casing 8 and a rotor 3 that is mounted in the casing 8 of the X-ray tube and positioned along the axis of symmetry 4, the rotor being mechanically fixed to the anode by means of a supporting shaft 25.
It is become increasingly common to use permanently operating X-ray tubes. This leads to ever greater levels of dissipated energy. To distribute the temperature of the anode more efficiently, it is made to rotate at ever higher rotation speeds, of the order of 10,000 rpm, in mechanical type tubes where the rotor is mounted on ball bearings. These ball bearings have to work under very difficult conditions for the temperature is great and the high vacuum of the tube in which they are placed prevents lubrication by means of a liquid.
Besides, a mechanical system such as this generates noises and vibrations that are detrimental to the practitioner's and patient's comfort as well as to the quality of the image obtained.
To overcome the drawbacks of mechanical bearing systems, it has been proposed to make devices for the magnetic suspension of the rotor-anode assembly: this prevents any mechanical friction.
It may be recalled that, to maintain the position of a cylindrical body 30 with a shape generated by revolution, having an axis O.sub.1 Z (FIG. 1), it is necessary to have control over five degrees of freedom along the axes, O.sub.1 X.sub.1, O.sub.1 Y.sub.1, O.sub.2 X.sub.2, O.sub.2 Y.sub.2 and O.sub.1 Z, with this body retaining the ability to rotate about the axis O.sub.1 Z.
According to Ernshaw's theorem, it is impossible to obtain the complete magnetic levitation of a body, i.e. magnetic levitation without any physical contact, only passively, i.e. through the use of permanent magnets only. This is why all the magnetic bearing devices proposed up till now to provide for the rotation of a rotor-anode assembly of an X-ray tube have at least one active path set up by means of an electro-magnet associated with an electronic circuit for positional servo-control along the axis corresponding to this active path.
It therefore becomes necessary to make a fairly complex active system to maintain, for example, the position of the rotor along the axis O.sub.1 Z. To this end, it is necessary to have position detectors: the information on positional deviation given by these detectors controls the value of the current in the electro-magnet so as to reduce this deviation.
In the French patent application No. 88 12483, filed on Sep. 23, 1988, a system has been proposed for the suspension of a rotating anode of an X-ray tube, of the type with passive magnetic bearings, wherein the maintaining of the longitudinal position of the rotor along the axis of rotation does not require the setting up of a complex servo-control system. The proposed system has (FIG. 2) at least two first magnetized rings, 5, 6 fixed to the rotor 3 and at least two other magnetized rings 9, 10 fixed to the casing 8 of the tube, said rings 5, 6, 9 and 10 being positioned in relation to one another so that the rotor 3 can rotate without friction about the axis of symmetry 4 and so that it is axially offset so as to come into contact with a stop 11 positioned on the side opposite that of the anode 22. The force of contact with the stop 11, due to the repulsion of the magnetic rings 5, 6, 9 and 10, should be as weak as possible when the anode is in an extreme angular position such that its weight moves it away from the stop 11. For any other position, the force of the contact will be greater and will have the drawback of increasing wear and tear at the points of contact of the stop 11.
This drawback is not bothersome when the axis 4 of the anode remains horizontal but becomes so when the axis 4 has to be inclined. This is the case in a scanner where the rotation plane of the tube, which corresponds to the plane of the patient's section that is to be displayed, tilts over to either side of the vertical plane, or in a mammography machine in which there is a complete tilting of the tube, with the anode being capable of taking a high position or a low position.